Continuous process for preparing uranium hexafluoride from uranium tetrafluoride andoxygen



Nov. 21, 1961 J. B. ADAMS ET AL 3,009,768

CONTINUOUS PROCESS FOR PREPARING URANIUM HEXAFLUORIDE FROM URANIUMTETRAFLUORIDE AND OXYGEN Filed Aprll 24, 1959 2 Sheets-Sheet 2 4 F U D EE F 0 a u|= TO UF6 RECOVERY zQEExo Y 4 m R a E F6V u w 555E w M 205336A2F6 O U moBfiE 2 of w mi N 2953? 0 Y 3 T R E F V H w 189mm D m n 2953mmF 2F H H mosfim F H m m m: zQZzEoadEE:

2 0 Y. O T R U F W w. w mosfim m E 292253 129: U m H H E H a moZfiE u 0RDA/EH6 E H D A 0 0 INVENTORS. Joseph B. Adams James C. Bresee BY LeslieM. Ferris Charles D. Scoff ---a W ATTORNEY United States PatentCONTINUOUS PROCESS FOR PREPARING URA- NIUM HEXAFLUORIDE FROM URANIUMTET- RAFLUORIDE AND OXYGEN Joseph B. Adams, James C. Bresee, Leslie M.Ferris, and Charles D. Scott, all of Oak Ridge, Tenn., assignors to theUnited States of America as represented by the United States AtomicEnergy Commission Filed Apr. 24, 1959, Ser. No. 808,856 9 Claims. (Cl.2314.5)

Our invention relates to the preparation of uranium hexafluoride andmore particularly to an improved method of converting uraniumtetrafluoride to uranium hexafluoride.

Large-scale conversion of UF to UP is currently eflected by reacting UF,with fluorine. This process presents serious disadvantages in thatelemental fluorine is both extremely expensive and difficult to handlebecause of its corrosiveness and toxicity. Consequently, it is desirableto prepare UF from UF without the use of fluorine.

One method of preparing UF from UF without the use of fluorine utilizesthe reaction of UF and oxygen at elevated temperatures as disclosed inUS. Patent 2,535,572. While small quantities of UP may be prepared byconducting this reaction in a tube-type reactor as disclosed in thispatent, the application of this process to large-scale equipment haspresented numerous difiiculties. UR, exhibits a strong tendency tosinter at the elevated temperatures, i.e., above approximately 750 C.,required for a reaction rate sufliciently rapid to allow highthroughputs in large-scale equipment. Merely contacting UR, and oxygenin conventional gas-solid reactors at these temperatures results in theformation of sintered agglomerates, thus drastically reducing thesurface area and the reactivity of the solids and plugging the reactor.In addition, the high temperature and high UR, content in the solids bedare favorable to the formation of large amounts of intermediatecompounds U F UP and U F which must be separated from the product UP andrecycled to the reactor. Low yields have resulted both because of thisUF sintering and intermediate formation and because of the adverseeffects of small amounts of moisture in the system. Another problempresented in conducting this reaction on a large scale is the provisionof a method of converting by-product uranyl fluoride to UR, andrecycling the UF Additional problems are presented in converting impureUF to UF UR, may be prepared directly from uranium ore concentrates byreduction and hydrofiuorination without employing the processes such assolvent eX- traction which are currently used for purifying oreconcentrates in large-scale production. Elimination of the need for thepurification process would result in substantial economic gain. Thisdirectly prepared UF however, may contain substantial amounts, e.g., upto percent, of various impurities such as sodium, iron or calcium. Thepresence of such impurities lowers the sintering temperature and meltingpoint of UF thus further increasing the sintering difiiculties in theUH,- oxygen reaction.

It is, therefore, an object of our invention to provide a large-scalemethod of preparing UF from UF without the use of fluorine.

Another object is to provide a method of reacting UF with oxygen inwhich the deleterious effects of UF sintering and the formation ofuranium-fluoride intermediates are minimized.

Another object is to provide a method of preparing UF from UF, in whichhigh yields are obtained.

Another object is to provide a method of recycling uranyl fluorideproduced in the reaction of UP, with oxygen.

Another object is to provide a method of converting impure UF to UPOther objects and advantages of our invention will be apparent from thefollowing detailed description.

In accordance with our invention, UF may be prepared from UF byintroducing finely divided dry UP; and a dry oygen bearing gas into abed of finely divided dry UO F under such conditions as to fluidize thereaction bed, maintaining the bed at a temperature from 600 C. to 900 C.until substantially complete conversion to UP and UO F is obtained andrecovering the UP The UO F bed functions as an inert solids diluent andreaction medium, with bed sintering and the resultant difficulties beingminimize-d by this means. The use of a UO F bed is particularlyadvantageous for a continuous process since UO F is a reactionby-product, being continually formed as the UH, is expended. Removal ofthe UO F may be readily accomplished by merely providing an overflow atthe top of the reaction bed. Fluorine is not required in this method,and the difficulties previously encountered in adapting the UP,- oxygenreaction to large-scale operation are minimized. Relatively high yieldsare obtained and by-product UO F may be reconverted to UF and recycled,thus providing an economical method of preparing large quantities of UFThe reaction of UF and oxygen proceeds as follows:

As may be seen from this equation, UO F is formed in large quantitiesduring the course of the reaction, thus necessitating recycle of thismaterial for high overall yields. Intermediate compounds UF U F and U Fmay also be formed as a result of side reactions which occur,particularly when product UF is exposed to high concentrations of finelydivided UF The side reactions are represented by the followingequations:

These intermediates, and in particular U F become entrained in theproduct gas stream in either solid or gaseous form and requireseparation. Upon return to the reaction bed, however, they decompose toyield UF and UF Thus, when the intermediates are continuously returnedto the bed at steady state operation, they will be present only in smallequilibrium amounts in the reactor system.

The UF -oxygen reaction is conducted in a conventional fluidized bedreactor initially charged with a bed of finely divided UO F whichfunctions as an inert solids diluent and reaction medium. Although UO Fdecomposes to yield UF U 0 and oxygen at temperatures above 800 C. in aninert sweep gas, we have found that decomposition is slight under theconditions employed in the present method, that is, in the presence ofUF and oxygen. In addition, any uranium oxide present in the bed reactwith UF to form UO F and UF The UO F bed may thus be maintainedthroughout the course of the reaction without difliculty from thissource.

In order to avoid the above-described sintering andintermediate-formation difficulties, the UR, concentration in the UO Fbed must be maintained at a relatively low level. A UF, concentration inthe bed of less than approximately twenty-five percent by weight isrequired, and a concentration less than approximately ten percent ispreferred. Higher concentrations are favorable both to sim tering andintermediate formation at practical operating temperatures. The UK;concentration in the bed may be controlled by adjustment of operatingvariables. UP, concentration is dependent on the feed rate and thereaction rate of the UF which in turn depends upon the temperature andother factors such as the concentration of oxygen in the oxygen-bearinggas. At a given feed rate an increase in the operating temperatureincreases the reaction rate and thus reduces the UP, concentration. Thereaction rate decreases and the UR; concentration increases with the useof a dilute oxygen-bearing gas such as air.

UF is introduced into the UO F bed by means of a solids-carrying gasstream, and an oxygen-bearing gas is concurrently introduced atsuflicient velocity to maintain the bed in a fluidized state. Thecomposition of the UH;- carrying gas is not critical, and any gas whichdoes not contaminate the. product or cause competing side reactions maybe employed. It is preferred to employ the sarne gas for carrying theUR, and for fluidizing the bed, with a major portion of the fluidizinggas being introduced in a separate stream. The oxygen-bearing fluidizinggas may comprise substantially pure oxygen or a diluted oxygen-bearinggas such as air. Because of the loss of product UP to by-product UO Fwhich occurs as a result of the presence of moisture in the system, itis essential that all reactant streams be dried and kept free ofmoisture.

The particle size of the initial UO F and the UK; feed is not criticalto our invention, and any material which is sufficiently finely dividedto be maintainable in a fluidized state may be employed. The particlesize to be used depends on the design of the particular reactorinvolved. For example, for a four-inch diameter fluidized bed reactor itis preferred to employ'initial UO F and UF feed in larger diameterreactors, where fluidization may be morereadily controlled. Largerparticles in general require excessive amounts of fluidizing gas.

The reaction of UF and oxygen is effected by maintaining the fluidizedbed at a temperature within the range of 600 C. to 900 C. Attemperatures below 600 C. the reaction rate is impractically low, andabove 900 C. sintering or melting tends to occur in the reaction bed.For substantially pure UF feed a temperature within the rangeof 750 C.to 850 C. is preferred. A slightly lower temperature, that is, from 700C. to 750 C., is preferred for UF containing impurities which tend tolower the UH, melting point. Although the method of heating the bed isnot critical, it is preferred to heat the initial U-O F by passing astream of a heated gas such as nitrogen through the bed to reach theoperating temperature and to maintain this temperature by preheating theinfluent gas streams to the bed temperature prior to introducing the gasinto the reactor. Maximum heat utilization may be obtained by firstcirculating the unheated fluidizinggas stream around the outside of thereactor at a point above the bed level. This serves both to cool theproduct gas as required for filtering and to supply a portion of theheat needed to bring the influent gas to operating temperatures.Conventional external heating coils may also be employed to maintain thedesired bed temperature.

Alternatively, a portion of the reaction heat may bev ootained bysupplying carbon monoxide to the bed in com bination with thefluidizingoxygenstream. Combustion of the carbon monoxide is catalyzedby solidswithin the bed so that heat is produced homogeneously throughout thebed, with heat transfer problems being minimized by this procedure. Highpurity carbon monoxide is required, since either moisture orhydrogen-containing gases which yield water vapor upon combustion lowerthe yield' of product UF In this alternative procedure the bedtemperature may be controlled by varying the amount of carbon monoxide.

a gas blowback system while the other filters-are'in operation Thechoice of gas forthis operationlis not critical cold traps to freeze outthe UF Filtration of the product gas stream is preferably conductedwithin the fluidized bed reactor by providing a baffle and a filterabove the level of the bed. The solid intermediates may be returned tothe bed, by merely cleaning the filter and allowing the solid particlesto fall. Filtrationmay also be effected in a separate vessel, butrecycle of the separated solids is rendered more difiicult. 'Forcontinuous operation, it is also preferred to use a plurality of filtersso arrangedthat one filter is alternately being cleaned-by an automaticas long as the gas is inert to UP and it is dry; The same gas as thefluidizing stream may be used. The filtered solids fall back into thebed and are converted to UR, and

UF In order to provide for effective removal of intermediates,filtration is conducted at an efliuent gas temperature belowapproximately 500 C., and preferably at about 200 C. At highertemperatures the uraniumfiuoride intermediates are -volatilized and thusare not amenable to removal by this means. The desired reaction andfiltration temperatures may be maintained within one vessel by employinga relatively long vessel, with the lower portion containing thefluidized bed being heated V and the upper or filtration section beingcooled, preferably by circulating the unheated influent gas streamaround the outside of the filter section. The filtered gas is thenpassed through conventional cold traps to freeze out the UF In order torecover trace amounts of UP the remaining gas is then passed throughchemical traps comprising beds of material such as calcium sulfate,alumina or sodium fluoride which react with or absorb and retain the UFSince UO F is produced'in the UH -O reaction, the quantity of thismaterial increases during operation. The excess UO F is readily removedand the bed maintained at a constant level by providing an overflow portat the top of the reaction bed.' I In order to provide a more economicaloverall yield, the UO F may be reconverted to UF and recycled to the UR,feed stream. Conversion of UO F to UF may be effected in a two-stepprocedure, comprising reduction of the UO F with hydrogen to yield amixture of U0 and UF and hydrofluorination of this mixture to convertthe U0 in the mixture to UF The f resulting UR, is then introduced intotheoxidation reactor UF feed stream. In the processing of impure UF aportion of the UO F is subjected to a purification step beforereduction. Purification may be readily effected by any conventionalmeans such as dissolution and precipitation,

dissolution and solvent extraction or dissolution, ion ex change andreduction.

Another recycle procedure may be employed which provides further overalleconomic gain by utilizing the offgases from the reduction andhydrofluorination reactions described above. In this procedure theseoff-gases, comprising HF, H and water vapor, are contacted with U0 feedmaterial to partially convert the U0 to U0 and UO F V The resultingmixture along with UO F byproduct from the oxidation reaction is thensubjected to reduction and hydrofluorination to provide further UF;

niercially under the trade name Inconel for components of the systemsubject to elevated temperatures such as the fluidized bed reactor whenthe pure UF is the feed material and to use nickel as the reactormaterial when the impure UR; is used. The nickel-base alloy availableunder the trade name Monel may be employed for lower temperaturecomponents, such as cold-trap and chemical-trap piping.

Apparatus suitable for conducting the method of our invention isschematically depicted in FIGURE 1. The UF -oxygen reaction is carriedout in a fluidized bed reactor 1 initially charged with a bed 2 of UO Fand provided with an external heater 2. Below the bed the reactor isprovided with an inwardly tapering portion 3, terminating in acylindrical gas inlet 4. An oxygen-bearing fluidizing stream isintroduced through the bottom of inlet 4 from an oxygen feed line 5. UR;entrained in a gas stream is introduced through an opening 6 in the sideof inlet 4, the UE; being fed from a hopper 7 through a solids meteringdevice 8 and line 9. The gas stream for carrying the UF is introducedfrom a gas feed line 10 through an inlet 11 into UR; feed line 9. Theinfiuent reactants are brought to the required temperature by means ofheaters 12, 12 on both the oxygen and UF feed lines. As the reactionproceeds the excess UO F is removed through an overflow port 13 into adownwardly extending overflow line 14. The UO F flows through a solidsvalve 15 and is collected in a container (not shown). Above the overflowport, the reactor is provided with an outwardly tapering portion 16terminating in a cylindrical filtering section 17. Entrained solidparticles are removed from the efliuent gas by means of a baffle 17' andsintered metal filters 18 positioned in the top 19 of the reactor. Thefiltered gas is removed through separate efiiuent gas lines 20 leadingfrom each filter to a single product line 21. A blowback system isprovided whereby each of the filters 18 is alternately cleanedautomatically while the other filters are in operation. This systemcomprises valves 22 in lines 20 for shutting ofi product gas flowthrough the filter being cleaned, and gas lines 23 provided with valves24 and leading from a blowback gas feed line 25 to effluent gas lines 29below the valves 22, together with suitable instrumentation (not shown).Product UP is recovered from the filtered efiluent gas by coldtrappingin cylindrical metal cold traps 26 connected in series in a container 27packed with coolant 28, which may suitably comprise a mixture of Dry Iceand trichloroethylene. The gas is introduced through a valved inlet line29 terminating near the bottom of the cold trap. The UP freezes out andthe remaining gas passes out of the trap through a valved outlet line 30at the top of the cold trap. Final traces of UP are recovered by passingthe gas through chemical traps 31 connected in series.

These traps comprise containers packed with beds of a suitable absorbentsuch as calcium sulfate. The gas is introduced through a valved inletline 32 at the bottom of the trap and removed through a valved outletline 33 at the top. The chemical trap efiluent gas is then discarded tothe atmosphere. Product UP is recovered from the cold traps and thechemical traps.

A system providing for recycle of UO F is schematically illustrated inFIGURE 2. Feed UF is converted to UP and UO F in an oxidation reactor 34of the type described with reference to FIGURE 1. UO F plus smallamounts of UR; are removed from the reaction bed by means of an overflowand are conveyed to a reduction reactor 35 for reduction with hydrogen.The reduction product, a mixture of UF and U is converted completely toUP; in a hydrofiuorination reactor 36 and recycled to the oxidationreactor feed stream.

Another recycle system which provides for utilization of off-gases maybe seen by reference to FIGURE 3. In this system U0 feed material iscontacted with hydrogen, HF and water vapor oil-gases from reduction andhydrofluorination reactors in an HF clean-up reactor 40. The

product of the clean-up reactor, a mixture of U0 UO F and U0 isconverted to UR; by reduction with hydrogen in a reduction reactor 38and with HF in a hydrofluorination reactor 39. The resulting UF is thenemployed as feed for the oxidation reactor 37. UO F overflows from theoxidation reactor and is converted to UF in the same manner as theclean-up reaction product.

Our invention is further illustrated by the following specific examples:

EXAMPLE 1 A UF oxidation run was conducted in a four-inch diameterfluidized bed reactor of the type described above with reference toFIGURE 1. The 4 inch section of the reactor was 24 inches long and wasoperated at bed depth of 13 /2 inches. The top of the reactor wasprovided with a six-inch diameter filter section containing foursintered metal filters and a baffie inserted between the bed and thefilters to serve as an impingement separator and to lower the filteringtemperature. The reactor was initially charged with 5,917 grams of UO Fand 12,620 grams of UF were fed into the bottom of the bed over a periodof 9.25 hours at an average feed rate of 1.37 kilograms UR; per hour.Air, dried in molecular sieve beds to a dew point of C., was employed asthe fluidizing gas and the UF -carrying gas. A bed operating temperatureof 810 C.:15 C. was maintained by preheating the influent gas streamsand supplying small amounts of heat through the walls. Steady-stateconcentrations of UF in the bed was 21.9 percent. The efiluent gas wasfiltered at a temperature of approximately 200 C., and the UF wasrecovered by passing the filtered gas through cold traps and chemicaltraps. The concentration of UP in the efiiuent gas was measured by meansof a condensation pressure analyzer. At the conclusion of the run, 5.12kilograms of UF was recovered from the cold traps. Further details andresults of this run may be seen by reference to the following table.

Table I CHANGE IN AMOUNTS OF INDIVIDUAL COMPOUNDS IN UF OXIDATION RUN*The amount of O entering the reactor was calculated to be thetheoretical amount necessary for oxidation in the various reactionsoccurring. The amount of UFs reported as collected is that amountcollected in cold traps and chemical traps and the amount of UFereported as measured is that amount of UFi measured in the reactoroil-gas by the condensation pressure analyzer. Material balance(material leaving/material entering):

98.1% using UFa collected. 100.4% using UFa measured.

The 5,610 grams of measured UF was 92.6 percent of the theoreticalamount produced and the recovered amount 85.4 percent. The remainder ofthe UF was consumed in side reactions with the reactor walls and withsmall amounts of uranium oxides and water in the system. The amount ofwater listed in Table I as entering the reactor was calculated from themoisture content of the feed. This run clearly demonstrates therelatively high yields obtainable by the method of our invention.

7 EXAMPLE 11 A UR oxidation run of longer duration Was'conducted in theapparatus used in Example I. 80.8 kilograms of UR; was fed into the bedover 58.4 hours at an average feed rate of 1.6 kg. UF per hour. Theoperating temperature was 800 C. to 825 C., with one brief excursion to775 C. Oxygen was employed as the UF carrying gas, the fluidizing gasand the blow-back gas. The steady-state concentration of UF in the bedwas maintained at 2.2 to 4.5 percent, except for the last'few hours ofthe run, when a mechanical failure in the UF feeder produced aconcentration of 46.4 percent. This high concentration of UF resulted insintering of some of the UE; in the bed, a build-up of solid materialson the walls and formation of large amounts of U4F1'1 which solidifiedin the upper portion of the reactor and restricted gas flow. Aftercorrection of the mechanical failure, the bed could not be effectivelyrefiuidized, thus demonstrating the adverse eflect of high UFconcentration in the bed. A total of 33.8 kilograms of UP was recoveredfrom the traps upon completion of the run. Other details and results ofthe run may be seen by reference to Table II.

Table II CHANGE IN AMOUNTS OF INDIVIDUAL COMPOUNDS IN UF OXIDATION RUN*The amount of O entering the reactor was calculated to be thetheoretical amount necessary for oxidation in the various reactionsoccurring.

'Ihe amount of HF leaving the reactor was calculated to be thetheoretical amount which resulted from the hydrolysis of UFo. Over-allmaterial balance (material leaving/material entering) =99.4%.

The amount of UP recovered in this run represents 90.4 percent of thetheoretical amount formed. The remainder of the UP was consumed in sidereactions, with 2.3 percent reacting with UF to form U4F17 due to thehigh UR; concentration brought about by the mechanical failure.

The above examples are merely illustrative and are not to be understoodas limiting the scope of our invention which is limited only asindicated in the appended claims. It is also to be understood that manyvariations in apparatus and procedure may be employed without departingfrom the scope of our invention.

Having thus described our invention, we claim:

1. The method of preparing UP from UF which comprises continuouslyintroducing finely divided dry UR; and a dry oxygen-bearing gas into abed of dry uranyl fluoride under such conditions as to fiuidize saidbed, maintaining said bed at a temperature within therange of 600 C. to900 C. and at a U1 concentration under approximately 25 weight percent,continuously removing the solids present in said bed from the top ofsaid bed and recovering the UP formed thereby.

2. The method of claim 1, in which said continuously removed solids aresubjected to reduction with hydrogen and. to hydrofluorination and theresulting UR, is reintroduced into said bed.

3. The method of claim 2 in which the ofi-gases resulting from saidreduction and said hydrofluorination are contacted with U0 to form asolids mixture comprising U0 U0 and UO F the resulting solids mixture issubjected to reduction and to hydrofiuorination and the UH; resultingthereby is re-introduced into said bed.

4. The method of preparing UF from UR; which comprises continuouslyintroducing finely' divided dry UF and a dry oxygen-bearing gas into abed of finely divided dry UO F in a vertical reactor under suchconditions as to fluidiz e said bed, maintaining said bed. at atemperature within the range of approximately 750 C. to 850 C. and at aUR, concentration under approximately 25 weight percent, continuouslyremoving the solids from the top of said bed, continuously subjectingthe resulting eflluent gas to filtration at a temperature underapproximately 500 C. and recovering UF from the resulting filtered gas.

5. The method of preparing UF from UR; containing UR; in a dry, finelydivided state and an oxygen-bearing gas into a bed of finely divided dryUO F under such conditions as to fiuidize said bed, maintaining said bedat a temperature within the range of 700 C. to 750 C. and at a UFconcentration under approximately 25 weight percent, continuouslyremoving the solids from the top ofsaid bed and recovering the UF formedthereby v 6. The method of preparing UF from UF which comprisesintroducing finely divided, dry UF a dry oxygenbearing gas and carbonmonoxide into a finely divided bed of UO F under such conditions as tofiuidize said bed, said carbon monoxide being continously burned in saidbed in an amount suificient to maintain said bed at a temperature withinthe range of 600C. to 900 C., said bed being maintained at a UR,concentration under approximately 25 Weight percent, and recovering theUF formed thereby.

7. The method of preparing UF from UF Which comprises introducing finelydivided dry UF and a dry oxygen-bearing gas into a'bed of finely divideduranyl fluoride under such conditions as to fluidize said bed,maintaining said bed at a temperature within the range of 600 C. to 900C. and at a UR; concentration under approximately 25 weight percent andrecovering the UF approximately 750 C.'to 850 C. and at a UFconcentration under approximately 10 weight percent, continuouslyremoving the accumulated solids from the top of said bed,continuouslyseparating entrained solid particles from the efliuent gasstream at a temperature below 500 C., returning said separated solidparticles tosaid bed and recovering UF from the resulting solid-freeeffluent gas stream.

9. The method of preparing UP from UF which'comprises introducing finelydivided, dry UF and a dry oxygen-bearing gas into the bottom of a bed offinely divided dry uranyl fluoride in a vertical reactor provided with areaction zone in the lower portion of said reactor and a filtration zoneat the top of said reactor under such conditions as to maintain said bedin a fluidized state, maintaining said bed in said reaction zone at atemperature within the range of approximately 750 C. to 850 C. and at aUF concentration under approximately 10 weight percent, continuouslyremoving the accumulated solids from the top of said bed,;continuouslyfiltering the ei'liuent gas stream in said filtration zone at atemperature under 500 C., returning the solids removed 9 by saidfiltration to said bed, and recovering UF from the resulting filteredefiluent gas stream.

References Cited in the file of this patent UNITED STATES PATENTS HainerDec. 26, 1950 Murphree Oct. 29, 1957 OTHER REFERENCES Fried et 211.: ABCReport AEC D-2981, May 1945.

Kirsles et al.: AEC Report, K567, March 15, 1950.

Perry: Chemical Engineers Handbook, 3rd ed. (1950), page 1577,McGraw-Hill Book Co., New York.

Moore: AEC Report TID 7501 (Pt. 1) pp. 33-51, February 1956, pub. by D.Van Nostrand, Inc., New York, 1959.

Ferris: AEC Report ORNL-2180, March 14, 1957.

7. THE METHOD OF PREPARING UF6 FROM UF4 WHICH COMPRISES INTRODUCINGFINELY DIVIDED DRY UF4 AND A DRY OXYGEN-BEARING GAS INTO A BED OF FINELYDIVIDED URANYL FLUORIDE UNDER SUCH CONDITIONS AS TO FLUIDIZE SAID BED,MAINTAINING SAID BED AT A TEMPERATURE WITHIN THE RANGE OF 600*C. TO900*C. AND AT A UF4 CONCENTRATION UNDER APPROXIMATELY 25 WEIGHT PERCENTAND RECOVERING THE UF6 FORMED THEREBY.